I've recently installed new cupboards in the kitchen and would like to install marble tile for the counter-tops. But some lady at color-tile said marble isn't the best choice... and of course I found some really nice marble that would look great?

I'm not an expert in the selection of either. But, in my experience of seeing marble on so many of my customer's counter-tops in their homes and their being very pleased with them, I don't see a problem with marble if it is what you want. Part of what I do for a living is that I am a handyman who does house painting and I am also a piano tuner. I have been in a lot of homes and the conversations with my customers have involved just about everything concerning their homes.

My suggestion is to remember that a sales person is a salesperson. Probably she might be trying to sell you something else where she might make more commission?

In my heart though, I would go with what is in your heart based upon an informed decision.

Hopefully, someone will come along who truly knows counter-top issues.

Neither are cheap , id preferr granite over marble mainly for the looks, granite is more expensive at around 55per ft, marble is slightly under that. For me marble is more for bathroom vanitys.
When I think of marble I think of 3/4 thick countertops, for a kitchen I like 1 1/4 or 1 1/2 thick countertops.
Its your house though get what you like , you've pay for it and live with it.